Two-component polymeric materials such as reactive adhesives, paints, gasket materials, and caulking materials comprise two separate components which react chemically with one another when intermixed. For example, two-component hot melt polymeric materials used in adhesive applications include a polymeric material and a second material such as a hardener. These types of hot melt adhesives, and other two-component polymeric materials, are dispensed from a system in which the two components are supplied in a predetermined ratio from separate metering pumps to a mixer/dispenser where they are intermixed with one another and dispensed onto a substrate. In such a system, if too much of one component is applied, then the characteristics of the combined mixture of such materials are undesirably altered. It is therefore important that the ratio of the components of two-component mixing and dispensing systems be exactly maintained. But this ratio is particularly difficult to maintain when the materials are supplied to a mixer/dispenser which is intermittent in operation, i.e., which is repeatedly turned on and off. In such applications, loss of ratio control characteristically occurs for a few seconds shortly after the dispenser valve is opened during which time a transient imbalance phenomena occurs caused by the elasticity in the system and the changing hydraulic pressures associated with cycling the mixer/dispenser.
Another problem which may occur in intermittent operation of two-component mixing and dispensing systems is a loss of flow control of the resulting mixture of the two components. It is desirous to control the flow rate of the resulting mixture dispensed to the substrate. However, during the first few seconds after opening of the mixer/dispenser, the transient imbalance phenomena described above may result in a loss of control of the flow rate of the mixture. Additionally, a loss of precise control of the flow rate of the mixture can occur as a result of changes in density or viscosity of either of the two components due to temperature changes thereof. If the mixture is an adhesive, this may result in less adhesive being applied to the substrate, which, in turn, may affect the bonding of materials.
The aforementioned problems with two-component mixing and dispensing systems have been addressed in U.S. patent application Ser. No. 07/640,060. As disclosed in that application, the exact ratio of the two components supplied to the mixer/dispenser can be controlled as a function of the pressure of the two materials at the inlets to the mixer/dispenser. It was recognized that maintenance of the desired ratio of such components requires adjustment and control of the pressure of each component at the mixer/dispenser during the off cycle of the operation. In order to achieve such control, the system of application Ser. No. 07/640,060 includes two back pressure controls, each of which is interposed between the intermittently operable mixer/dispenser and a metering gear pump connected to a source of one of the components of liquid material. Each back pressure control comprises a bypass flow path around each metering pump and an adjustable pressure regulator contained in that bypass path. Additionally, each bypass flow path includes a flow control valve which is closed when the dispenser flow control valve is open and vice versa. To control the pressure of each component at their respective inlets to the dispenser, the adjustable pressure regulator in each bypass flow path is operated either manually or automatically to adjust the pressure at the inlets to the mixer/dispenser when the flow control valve is closed so that such pressure is equal to or a function of the steady state flow pressure at the inlet to the mixer/dispenser when its valve is open.
Despite the above-mentioned improvements in the control system associated with two-component mixing and dispensing systems disclosed in U.S. patent application Ser. No. 07/640,060, additional problems have been discovered with intermittent operations of systems of this type. As described above, the two components are supplied separately from individual sources into the mixer/dispenser where they are combined with one another to form a mixture prior to deposition onto a substrate. Within a short period of time, these intermixed materials begin to cure within the mixer/dispenser which can appreciably increase the viscosity of the mixture and/or create problems of plugging or clogging of the mixer/dispenser with cured material. Depending upon the period of time during which the mixer/dispenser is shut off, and other parameters such as temperature of the components and the ratio at which such components are supplied to the mixer/dispenser, a pressure substantially in excess of the steady state pressure may be required to obtain the desired volumetric or mass flow rate of each component, and therefore the ratio and the total flow rate of the mixture from the mixer/dispenser when its valve is again opened and flow is resumed.